The invention relates to a superconducting level measuring apparatus for liquid hydrogen that can be used in the several application cases of storage, generation and consumption of liquid hydrogen. The superconducting level measuring apparatus is further usable for liquid neon. The invention further relates to a measuring method for measuring a liquid level.
Diverse methods and devices for measuring the level of cryogenic liquids in vessels, based on the most varied concepts, are quite generally known in the prior art.
DE 102 58 235 A1 discloses a level measuring apparatus for a vehicular liquid gas tank, in particular a vehicular liquid hydrogen tank, which has a wire strain gage arranged on the suspension of the inner tank and a pressure sensor arranged outside the vehicular tank and connected to the vehicular tank via a line, the strain gage and the pressure sensor each being connected to an on-board computer via a line. According to the above-cited publication, the level can be inferred indirectly by computational means from the measured total weight of the inner tank and the associated pressure. Doing so requires suitable software and calibration of the system.
This method further has the disadvantage that the side effects, in particular on the weight measurement, are so great that no reliable determination of the level is possible. The inaccuracies that arise, for example through continuously varying acceleration during driving, lead to interferences that make it impossible to determine the level reliably.
Further known in the prior art is the use of probes based on discrete sensors, wherein the presence of a gaseous or liquid phase can be established for the individual sensors. An inherent disadvantage here is the limitation to discrete measuring points; that is, the level necessarily cannot be tracked continuously. The generally large number of individual sensors required occasions a high expense; a complex control device is necessary, so that this method can be used to advantage only in a few cases.
The principle of the capacitive level probe has been technically implemented and is now widespread. Disadvantageous here are the large size and heavy weight of the probes and, what is more, the stringent requirements on manufacturing tolerances and dimensional stability in fabrication and later operation. Relatively low signal levels in the pF range must be employed, making the method costly in terms of apparatus and susceptible to interference. Further, the level actually to be determined can only be calculated indirectly from the measured capacitance and other parameters such as pressure and temperature; there are numerous interfering effects that make interpretation laborious and inaccurate. The difficulties stated constitute a substantial limitation on the current use of this method in the vehicular field.
Known from the prior art are level measuring apparatuses for other cryogenic liquids, in particular for liquid helium, which are based on the use of a superconducting material. The principle employed here is the inequality in heat transfer between the gas phase on the one hand and the liquid phase on the other. The superconducting material here is generally fashioned as a longitudinally extended conducting path in substantially vertical orientation. The transition temperature must be slightly above the boiling point of whatever cryogenic fluid is to be measured. The method rests on the fact that the superconducting material in the gas phase is held above the transition temperature, that is, in the normally conducting state, by deliberate input of heat. The portion of the superconductor located in the liquid phase remains in the superconducting state because of better cooling. The position of the liquid level can be inferred by measuring the electric resistance. Such superconducting level probes are described, for example, in DE 26 15 407 and U.S. Pat. No. 3,943,767. A fundamental description can be found in Efferson, K. R., “A Superconducting (Nb—Ti) Liquid Helium Level Detector,” in Timmerhaus (ed.), Advances in Cryogenic Engineering, Plenum Press, Vol. 15 (1970), p. 15.
Further, WO 91/08449 discloses a level detector for cryogenic liquids, in particular liquid nitrogen, wherein the liquid level is detected through the use of the superconductivity effect with a superconductor from the group of high-temperature superconductors. A superconducting film is applied to a support with an intervening substrate such as an yttrium-stabilized ZrO2 substrate. In a detection circuit, the changes in electric resistance in the superconducting material are acquired by a low-frequency sampling method and conditioned, a measuring current being routed through the superconducting film in pulse-clocked fashion and calibration pulses for the measuring circuit being provided between the measurement pulses.
The known level measuring devices using this principle are not, however, usable for liquid hydrogen. This is so because of the material properties of liquid hydrogen, which have heretofore blocked any use of this method.